With the development of planing-type hulls and surface-drive propulsion systems, a problem arises when a designer attempts to combine the two performance enhancing features. Namely, prior to the development of the present invention, there was no inboard propulsion system where the propeller shaft passed directly from the transmission through the boat hull bottom ahead of the transom thereafter presenting a screw propeller partially above the running water line behind the transom. Solutions taught by the prior art either use jointed propeller shafts or pass the propeller shaft directly through the transom. Both of these solutions are inherently highly complicated and expensive.
An appreciation of the contribution of the present invention to the art of boat design comes with an understanding of the two performance enhancing features that the present invention allows a designer to combine. Namely, the present invention allows the boat manufacturer to combine a planing-type boat hull with a surface-drive propulsion system.
It is known that boats having planing hulls operate more efficiently at speeds where the hull "planes" on top of the surface of the water with a minimum area of the hull in contact with the water. The improved performance arises from the decreased drag attributable to the propeller operating partly above the water. Minimizing drag increases speed and fuel efficiency. Typically, about one-half to two-thirds of a planing hull is in contact with the water surface at planing speeds. With "V" shaped hulls, only the area proximate to the keel is in contact with the water surface during planing. However, the wetted area of the hull varies depending upon the design and speed of the vessel. As much as three-quarters of the hull area may be wetted at lower speeds, and as little as one-fourth of the hull area may be wetted during very high speed cruising. Hull designs of this type, that allow the boat to plane, are known in the art as planning-type hulls.
The second design feature successfully utilized by this invention is the surface-drive propulsion system for an inboard motor boat. When a propeller is presented partially above the extended plane of the bottom of the boat, a portion of the propeller is above the "running water line" when the boat attains the planing condition. By presenting a portion of the propeller above the running water line (and therefore above the surface of the water) during the planing condition, a more effective propeller thrust is transmitted to the boat resulting in additional boat speed without increased use of engine power. The accepted physical explanation for the increase in the propeller's thrust efficiency when operating partially above the running water line is that there is less peripheral water friction operating on the propeller blades as the portion of those blades are no longer continuously submerged under the surface of the water.
Regardless of the exact physical explanation for the above phenomena, surface-drive propulsion systems are effective in increasing boat speed without using additional engine power. The combination of a planing hull and a surface-drive propulsion system provide an effective and efficient use of a boat engine's power.
For boats with inboard motors, the surface-drive propulsion system has been limited in its application because an economical design of a propeller shaft extending from an inboard motor that presents a portion of the propeller above the running water line was not available. The three designs currently employed are all inherently impractical to build and maintain.
First, a surface-drive system can be employed with an inboard motor when the propeller shaft passes through the transom. This design scheme is impractical and expensive because piercing the transom with the propeller shaft requires specially fabricated hardware including shaft parts, radial thrust bearings and special packing glands. Because specially designed hardware is required to pass a propeller through the transom and these parts must be maintained and replaced during normal maintenance, this design is not only expensive initially but is also expensive to maintain and operate.
Second, in order to avoid passing the propeller shaft through the transom, manufacturers have designed propeller shafts with special universal joints and special couplings thereby creating a jointed shaft that extends through the hull of the boat and then angles rearward in a parallel fashion in order to present the propeller blades partially above the keel and running water line. This design scheme is also expensive and impractical. The special universal joints and couplings required between the propeller and transmission are expensive to manufacture and also expensive to maintain.
Finally, a third design employs one or two inboard engines mounted near the transom with an output shaft extending forward toward the bow. A double universal joint coupling mechanism connects the forwardly extending output shaft to two rearwardly extending propeller shafts. The dual rearwardly extending propeller shafts extend through the rear of the hull without penetrating the transom. However, the dual propeller shafts and special couplings associated with this design also suffer from high initial investment costs and high maintenance costs.
Thus, there is a need for a boat with an inboard motor that employs a single, straight conventional propeller shaft that extends through the bottom of the hull and still presents a portion of the propeller above the running water line, all in combination with the planing-type hull. Only in this manner can the two aforenoted design features, namely planing-type hulls and surface-drive propulsion systems, be combined with inexpensive inboard boat designs and standard marine hardware.